<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .item {
      color:red;
    }
    .back {
      background-color: blue;
    }

  </style>
</head>

<body>
  <ul>
    <h1>标题</h1>
    <li class="item">1</li>
    <li class="item">2</li>
    <li class="item">3</li>
  </ul>
  <script>
   // style
   // className
   // classList
   // setIntervel
   // clearIntervel
  </script>
</body>

</html>